Pharmacologic differences between beta blockers.

All of the beta blockers act by antagonizing the actions of the endogenous adrenergic agonists epinephrine and norepinephrine at the beta-adrenergic receptors. However, a number of pharmacologic differences exist between the various agents. Some drugs, such as atenolol and metoprolol, are relatively selective for the beta-1-adrenergic receptors, requiring higher concentrations to block beta-2-adrenergic receptors than are required to block beta-1 receptors. It should be noted, however, that these selective beta blockers all block beta-2 receptors when their concentrations are high enough. When patients with asthma must receive a beta blocker, low doses of a selective drug should be used. Recent studies, however, have suggested that the use of a nonselective beta blocker may be desirable to antagonize some beta-2-mediated metabolic effects, such as hypokalemia, induced by epinephrine. Pindolol is the only beta-receptor antagonist available in the United States with intrinsic sympathomimetic, or partial agonist, activity. Such drugs, because of their partial agonist activity, cause some sympathetic stimulation under conditions of low endogenous sympathetic tone, such as while subjects are at rest in the supine position. Under conditions of higher sympathetic tone, pindolol blocks the effects of the endogenous agonists, producing the characteristic effects of a beta blocker. Membrane-stabilizing activity was first recognized with propranolol, and the value of this property has been a source of controversy ever since, but recent studies suggest that propranolol may induce electrophysiologic effects by mechanisms other than beta blockade. Pharmacokinetic differences between the drugs are also of importance.(ABSTRACT TRUNCATED AT 250 WORDS)